Depth distribution and depth adaptation of microbiomes in juvenile and adult scleractinian corals (Pocillopora verrucosa) in the central South China Sea

Environmental conditions at different water depths on tropical atolls are substantially different, but few studies have investigated the depth distribution and adaptation in the microbiomes of juvenile and adult corals. We collected samples of juvenile and adult Pocillopora verrucosa from different depths on an atoll in the central South China Sea and analyzed their communities of symbiotic Symbiodiniaceae and associated bacteria. Results showed that the Symbiodiniaceae communities were significantly different at different depths in both juvenile and adult P. verrucosa. Symbiodiniaceae community changed with increasing depth and was dominated by Durusdinium and Cladocopium at 1-m and 20-m depth, respectively. Furthermore, the composition and diversity of Symbiodiniaceae in juvenile and adult corals were different at the same depths. The compositions of associated bacterial communities at different depths also differed significantly between juvenile and adult P. verrucosa. The relative abundance of Endozoicomonas in juvenile and adult P. verrucosa increased with increasing depth, while the Thermus decreased. Our study suggests that juvenile and adult P. verrucosa corals in shallow-water regions associate with high abundances of thermotolerant Symbiodiniaceae and bacteria in the shallow reef flats with high temperature and intense solar radiation, but associate with Symbiodiniaceae and associated bacteria with high photosynthetic efficiency at greater depths.